Typically a laser weapon needs to perform three primary functions. It must search for, detect, and identify targets. This is usually called the search and acquire mode which utilizes a wide field-of-view. After the target is acquired it is handed over to a tracking system that utilizes a narrow field-of-view. The target is tracked and an aim point is designated. At this time the negation laser can be fired to destroy the target. Then, the system switches back to the search and acquire mode to repeat the sequence of these three functions. To perform these functions requires two or more lasers. The search and track lasers (which may be the same laser) are low power cw lasers and the negation lasers is a high energy pulsed laser. Thus it is necessary for the track laser, which designates the aimpoint on the target, and the high energy negation laser to be aimed at the same point. This is generally accomplished by having these two lasers share the same exit aperture which means that they must be boresighted.
Obtaining and maintaining boresight is a difficult task and complicated systems have been designed for this purpose. As an example, consider FIG. 1 which illustrates such a system. Here the output search and track laser 1 is reflected off a local loop tilt mirror 2. Part of this beam is now reflected by beam splitter 3. This part of the beam is then chopped by chopper 4 and is reflected back on itself by the corner cube 5. After reflection it passes through the beam splitter 3 and is focused by the lens 6 onto the beam transport alignment sensor 7. The remaining portion of the beam is propagated to the beam splitter 8. Another part of the beam is reflected off this beam splitter 8 and is propagated through the hole in the boresight mirror 9 to the grating 10. The zeroth order reflection off the grating 10 is propagated to mirror 11 where it is reflected to the beam transport tilt mirror 12. The beam is then transported to the gimbal mirrors (not shown) where it is reflected to output telescope secondary mirror 21 and a small part of the energy is reflected from the secondary back towards the beam transport tilt mirror 12 where it is made to retrace itself by adjusting the tilt mirrors 12 and laser 1 until both the transmitted beam and the reflected beam are imaged to the same spot on the sensor 7. At this point the search and track laser has been aligned. The first order reflection of the search and track laser beam off the grating 10 is reflected by mirror 13 to the diagnostic tilt mirror 14 and after reflection from this mirror it is imaged by the lens system 15 on the boresight sensor 16. The diagnostic tilt mirror is used to position this beam on the sensor 16. The portion of the search and track laser beam that passes through the beam splitter 8 is reflected by mirror 17 through the chopper 18 to the tilt mirror 19. This beam is then sent through the resonator 20 of the negation laser. When the beam leaves the resonator of the negation laser it has a large diameter and it is hollow. This beam is reflected from the boresight mirror 9 to the grating 10. The boresight mirror 9 is now tilted until the first order reflection from the grating 10 is imaged by mirrors 13 and 14 and the optical system 15 onto the same spot on the boresight sensor 16 of that where the search and track laser's first order reflection is imaged.
Once this boresight alignment has been obtained, control loops on all the tilt mirrors are required in an attempt to maintain this alignment.